Intermediate workpiece employing a mask for etching an aperture aligned with the crystal planes in the workpiece substrate

ABSTRACT

A method for differentially etching an N-sided polygon aperture through a first major surface of a &lt;100&gt; silicon wafer along the &lt;111&gt; planes begins with depositing a mask and defining therein a first intermediate polygon aperture having at least 4N+2 sides, where N is a positive integer. At least one side is generally parallel to the &lt;110&gt; plane, and the intersection of a second side and a third side of the first intermediate polygon is located generally along a major crystal axis perpendicular to the &lt;110&gt; plane. The included angle between the second and third sides expands during anisotropic etching to form one of the N sides of the polygon located along the major axis perpendicular to the &lt;110&gt; plane.

This is a divisional of application Ser. No 08/160,531 filed Dec. 1,1993 now Pat. No. 5,484,507.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to etching features into a semiconductorsubstrate, and in particular to a method and workpiece for preciselyetching an aperture of known cross-sectional area through the substrate.

2. Description of the Prior Art

Injector nozzles for ink jet printers and fuel injectors for internalcombustion engines require precise metering of injected fluids, which inturn requires a precise aperture of know cross-sectional area and shape.While silicon micromachining technology allows for relatively precisecontrol of the etching process for the aperture, improved accuracy isoften not realized because of the misalignment of the pre-etch maskopening with the crystal lattice. While a circular mask would alleviatethe requirement to align to the crystal lattice, computer generatedmasks can not be generated with a perfect circle. Any polygon used as anapproximation for the mask aperture will have straight sides that aremisaligned from the crystal lattice because of the limited accuracy oftools used for aligning the mask and the substrate material.

The crystal orientation of the silicon wafer causes a rectangularaperture to be generated by the etching process regardless of thepolygon used in the pre-etch mask pattern. If the original mask apertureis chosen as a rectangle or square, then the sides of the polygon mustalign exactly with the <111> planes of the wafer. Any misalignmentresults in the final aperture being a square having the largestdimensions in each <111> direction of the original, misaligned squareaperture in the pre-etch mask. See FIG. 3 for a visual representation ofthe rotation of the pre-etch mask aperture and the process by which itcreates an aperture of enlarged cross-sectional area in the siliconsubstrate.

Poteat, in U.S. Pat. No. 4,470,875 discloses a method for constructingan alignment indicator in the silicon wafer to aid in the process ofaligning the aperture in the pre-etch mask more precisely with thecrystal lattice within the substrate. However, the Poteat process doesnothing to improve the mechanical misalignments inherent in theequipment used for aligning the pre-etch mask with the silicon wafer,even when the precise crystal orientation is known. This misalignment isespecially significant when it is necessary to etch cooperatingapertures through the substrate from opposite sides of the wafer. It istherefore an object of the present invention to define a process foreliminating the enlargement introduced in an aperture etched in asubstrate when the aperture in the pre-etch mask is misaligned withrespect to the crystal lattice in the substrate.

SUMMARY OF THE INVENTION

A process for removing material from and defining an N-sided polygonaperture of known cross sectional area through a first major surface ofa wafer of crystalline material, with the first major surface includingtherein first and second major crystal axes, begins with depositing onthe first major surface a first pre-etch mask defining therein a firstintermediate polygon aperture having at least 4N+2 sides where N is apositive integer. At least one side of the first intermediate polygon isgenerally parallel to the first major crystal axis, and the intersectionof a second side and a third side of the first intermediate polygon islocated generally along the second major crystal axis. The first majorsurface of the crystal is anisotropically etched through the firstintermediate polygon aperture in the first mask for defining the N-sidedpolygon aperture in the substrate. The included angle between the secondand third sides is expanded through the anisotropic etching process tobecome another of the N sides of the polygon located along the secondmajor crystal axis, thereby defining the N-sided polygon having a knowncross-sectional area. The first mask then may be removed from the firstmajor surface as required.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present invention may beobserved through a study of the written description and the drawings inwhich:

FIG. 1 is a bottom view of a fuel injector nozzle etched into a siliconsubstrate.

FIG. 2 is a sectioned view taken along cross-sectioned lines 2--2through the center of the fuel injector nozzle of FIG. 1.

FIG. 3 is a top conceptual illustration of the misaligned aperture inthe mask on the silicon substrate, and the resulting enlargement of theetched nozzle.

FIG. 4 is a top view of a polygon having 4N+2 sides defined in the maskon the silicon substrate prior to the etching process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A typical fuel injector nozzle 10 is illustrated in FIGS. 1 and 2 ascomprising a square or nearly square aperture 20 in the upper surface 11of a silicon substrate 14. The silicon substrate in the preferredembodiment is typically a single crystal silicon wafer and the uppersurface 11 is generally in the <100> crystal plane. Aperture 20 isdefined by first, second, third and fourth edges, 21, 22, 23 and 24respectively.

The four sides, illustrated as 31, 32, 33 and 34, of the nozzle 10 aregenerally located along the <111> crystal plane in the substrate 14. Thefour sides terminate at a lower surface 12 of the substrate 14 at edges41, 42, 43 and 44 respectively.

Edges 21 and 41 are generally parallel to each other and to the <110>crystal plane. Edges 23 and 43 are generally parallel to edges 21 and41, and also to the <110> crystal plane. Edges 22 and 42 are generallyparallel to each other and to the <110> crystal plane, which in asilicon crystal lattice is generally perpendicular to the <110> crystalplane.

The dimensions of the upper aperture 20 are critical because the crosssectional area of the aperture 20 serves to meter the fuel ejected underpressure therethrough. While the size of the lower aperture 30 isimportant, it does not significantly affect the operation of the fuelinjector.

FIG. 4 illustrates an aperture 60 defined in a first silicon dioxide(SiO₂) pre-etch mask 40 covering the upper surface 11 of the siliconsubstrate 14. The aperture is formed from thirty flat sides or facetsdefining a polygon having a central axis that is generally perpendicularto the upper surface 11 and the <100> crystal plane therein. The x and ydimensions of the aperture 60 as defined in FIG. 4 illustrate that theaperture 60 is a close approximation to a circle. However, it isimportant to note in FIG. 4 that the upper and lower facets at theextremes of the Y dimension are actually coincident with and generallyparallel to the 110 direction, while the facets at the extremes of the Xdimension form an angle whose bisecting line is parallel to the <110>plane and the 110 direction, which in silicon is generally perpendicularto the <110> crystal plane. This geometry may be represented by definingthe number of facets forming the aperture in the pre-etch material as4N+2, where N is a positive integer. This ensures that the number ofsides in the aperture will always be divisible by 4 while leaving aremainder of two sides. This remainder always assures that the Xdimension extremes will be the intersection of two sides and will not beparallel facets.

As illustrated in FIG. 4, the X dimension is measured in a directionperpendicular to the intersection of the <100> and <110> crystal planes.This direction can be said to define a first major crystal axis.Likewise, the Y dimension is measured in a direction parallel to theintersection of the <100> and <110> crystal planes, which intersectionand direction can be said to define a second major crystal axis. Insilicon, the first and second major axes are perpendicular to andintersect with each other and they both lie within the <100> crystalplane as illustrated in FIG. 1.

With this construction for the aperture 60, a differential oranisotropic etching process using potassium hydroxide and water willcause the <111> plane to change dimensions so slowly that the locationof the facet actually functions as an etch stop, while the much fasteretching in the <110> crystal plane effectively causes the round aperture60 to etch into a generally square aperture 20 in the upper surface 11of the silicon substrate 14. Any inadvertent rotation of the aperture 60in the mask 40 from the orientation shown in FIG. 4 will cause the Xdimension to shrink while the Y dimension will increase within the mask40, and these self compensating changes will result in the area definedby the aperture 60 being essentially unchanged. This self-compensatingfunction provides fuel injector nozzles having the same size opening,even when used in a mass production process, which therefore willdeliver the same volume of fuel under pressure as the ideal apertureconstruction.

A process for manufacturing the silicon micromachined nozzle would beginwith a cleaning of the wafer, and then an initial orientation of thewafer with respect to the flat circumferential section which typicallylies in the <110> crystal plane and establishes the <110> direction. Thepre-etch mask 40, such as SiO₂ or silicon nitride, is formed on theupper surface 11 of the substrate 14. The polygon-shaped aperture 60having 4N+2 sides is then formed at the desired location in the pre-etchmask by standard photolithographic and etching techniques. The siliconsubstrate is then anisotropically etched such that the <100> and the<110> crystal planes etch at a much faster rate than in the <111> plane.This differential etching will cause the silicon beneath the pre-etchmask to be removed along the <111> crystal plane as illustrated in FIG.2. The duration of the etching process can be regulated so that thedepth of the aperture within the substrate may be controlled. In thefirst preferred embodiment, the aperture is etched through the siliconsubstrate, and the etch time is sufficiently long to allow full etchingof the corners of the square aperture in the upper surface of thesubstrate as illustrated in FIG. 1. The pre-etch mask is then removedusing conventional processing techniques.

While in the first preferred embodiment it was assumed that the lowersurface 12 of the substrate 14 was protected from the etching processwith a mask, it is possible to mask and etch another aperture from thebottom side upward through the substrate. If the two apertures arecarefully aligned, then the square aperture cross-sections can be madeto be congruent at the point where the apertures join.

While the invention has been described in conjunction with theprocessing of a square aperture in a silicon substrate in the <100>plane, it should be apparent that other aperture shapes can bemanufactured into other crystalline substrates, such as galliumarsenide, lithium niobate or indium phosphide, without departing fromthe scope and spirit of the present invention. While the preferredembodiments of the present invention have been explained herein, variousmodifications will be apparent to those skilled in the art. All suchvariations are considered to be within the scope of the followingclaims.

I claim:
 1. An intermediate workpiece used in a process for removingmaterial from and defining an N-sided polygon aperture of known crosssectional area through a first major surface of a silicon wafer alongthe <111> plane therein, with the first major surface oriented in the<100> plane, comprising in combination:a first mask covering the firstmajor surface of the silicon wafer and defining therein a firstintermediate polygon aperture having at least 4N+2 sides, where N is apositive integer, with at least one side of said first intermediatepolygon being generally perpendicular to the <110> plane, and with theintersection of a second side and a third side of said firstintermediate polygon being located generally along a major crystal axisparallel to the <110> plane.
 2. The workpiece as described in claim 1,further including:a second mask, covering a second major surface of thesilicon wafer opposite to the first surface, for defining therein asecond intermediate polygon aperture having at least 4N+2 sides, with atleast one side of said second intermediate polygon being generallyperpendicular to the <110> plane, and with the intersection of a secondside and a third side of said second intermediate polygon being locatedgenerally along a major crystal axis parallel to the <110> plane.
 3. Theworkpiece as described in claim 2 wherein said 4N+2 sides form ahexagon.
 4. The workpiece as described in claim 2 wherein a central axisof said first intermediate polygon aperture of said first mask isgenerally coextensive with a corresponding central axis of said secondintermediate polygon aperture of said second mask.
 5. An intermediateworkpiece used in a process for removing material from and defining anN-sided polygon aperture of known cross sectional area through a firstmajor surface of a wafer of crystalline material, with the first majorsurface including therein first and second intersecting major crystalaxes, comprising in combination:a first mask deposited upon the firstmajor surface and defining therein a first intermediate polygon aperturehaving at least 4N+2 sides, where N is a positive integer, with at leastone side of said first intermediate polygon aperture being generallyparallel to the first major crystal axis, and with the intersection of asecond side and a third side of said first intermediate polygon aperturebeing located generally along the second major crystal axis, whereby theincluded angle between said second and third sides of said firstintermediate polygon aperture will expand through anisotropic etching toform another of the N sides located along the second major crystal axisfor defining the N-sided polygon aperture.
 6. An intermediate workpieceused in a process for removing material from and defining a firstN-sided polygon bore of known cross sectional area through a first majorsurface of a silicon wafer along the <111> planes therein, with thefirst major surface oriented in the <100> plane, comprising incombination:a first mask deposited upon the first major surface anddefining therein a first intermediate polygon aperture having at least4N+2 sides, where N is a positive integer, with at least one side ofsaid first intermediate polygon being generally perpendicular to the<110> plane, and with the intersection of a second side and a third sideof said first intermediate polygon being located generally along a lineparallel to the <110> plane, whereby the included angle between saidsecond and said third sides of said first intermediate polygon aperturewill expand during anisotropic etching to form one of the N sideslocated along the major axis parallel to the <110> plane.
 7. Theintermediate workpiece as described in claim 6 wherein said firstintermediate polygon aperture comprises a hexagonal aperture, with atleast one side thereof parallel to the <110> axis, and with theintersection of a second side and a third side thereof located generallyalong a major crystal axis perpendicular to the <110> axis, wherebyanisotropic etching will enlarge the included angle between said secondside and said third side to form one of the N sides located along themajor axis perpendicular to the <110> axis.
 8. The intermediateworkpiece as defined in claim 6, further including:a second maskdeposited upon a second major surface of the intermediate workpiece anddefining therein a second intermediate polygon aperture having at least4N+2 sides, with at least one side of said second intermediate polygonaperture being generally perpendicular to the <110> plane, and with theintersection of a second side and a third side of said secondintermediate polygon aperture being located generally parallel to the<110> plane, whereby the included angle between said second and thirdsides will expand during anisotropic etching to form one of the N sidesoriented generally parallel to the <110> plane.
 9. The intermediateworkpiece as described in claim 8 wherein said second intermediatepolygon aperture comprises a hexagonal aperture, with at least one sidethereof parallel to the <110> axis, and with the intersection of asecond side and a third side thereof located generally along a majorcrystal axis perpendicular to the <110> axis, whereby anisotropicetching will enlarge the included angle between said second side andsaid third side to produce one of the N sides located along the majoraxis perpendicular to the <110> axis.
 10. The intermediate workpiecedescribed in claim 8 wherein a central axis of said first intermediatepolygon aperture in said first mask is aligned with a central axis ofsaid second intermediate polygon aperture in said second mask foraligning a generally overlapping intersection of the first and secondN-sided polygon bores through the silicon wafer.
 11. An intermediateworkpiece used in a process for removing material from and defining afirst generally rectangular bore, having a known cross sectional areaand defined along the <111> planes, through a first major surface of asilicon wafer oriented in the <100> plane, comprising in combination:afirst mask shielding the first major surface and defining therein afirst intermediate polygon aperture having at least 4N+2 sides, where Nis a positive integer, with at least one side of said first intermediatepolygon aperture being generally perpendicular to the <110> plane, andwith the intersection of a second side and a third side of said firstintermediate polygon aperture being located generally along a lineparallel to the <110> plane, whereby the included angle between saidsecond and third sides of said first intermediate polygon aperture willexpand during anisotropic etching to form an edge of the rectangularbore oriented generally parallel to the <110> planes.
 12. Theintermediate workpiece described in claim 11, wherein said firstintermediate polygon aperture comprises a hexagonal aperture, with atleast one side of said hexagonal aperture being generally perpendicularto the <110> plane, and with the intersection of a second side and athird side of said hexagonal aperture being located generally along aline parallel to the <110> plane.
 13. The intermediate workpiece asdescribed in claim 11 further including a second mask shielding a secondmajor surface of the silicon wafer oriented in the <100> plane, withsaid second mask defining therein a second intermediate polygon aperturehaving at least 4M+2 sides, where M is a positive integer, with at leastone side of said second intermediate polygon aperture being generallyperpendicular to the <110> plane, and with the intersection of a secondside and a third side of said second intermediate polygon aperture beinglocated generally along a line parallel to the <110> plane, wherein theincluded angle between said second and third sides of said secondintermediate polygon aperture will expand during anisotropic etching toform an edge of the second rectangular bore oriented generally parallelto the <110> planes.
 14. The intermediate workpiece as described inclaim 13 wherein an axis of said first intermediate polygon aperture insaid first mask is aligned with an axis of said second intermediatepolygon aperture in said second mask for creating an intersection ofsaid first and second intermediate polygon apertures as anisotropicetching progresses.